1. Field of the Invention
The present invention relates to a polyamic acid varnish composition which is intended to form a polyimide insulating layer of a flexible printed board, and a flexible printed board which employs same.
2. Description of the Related Art
Flexible printed boards in which polyimide insulating layers formed directly onto a metal foil, e.g., copper foil, without an adhesive being interposed therebetween are manufactured by applying a polyamic acid varnish composition, which is obtained through the addition polymerization of an aromatic diamine such as a paraphenylenediamine and an aromatic acid dianhydride such as pyromellitic dianhydride in a solvent such as N-methyl-2-pyrrolidone, onto the copper foil, drying the whole to obtain a polyamic acid layer (polyimide precursor layer), and then heating and imidizing it at 300-400xc2x0 C. to form a polyimide insulating layer.
However, the presence of carboxyl groups in the polyamic acid gives rise to such problems as the corrosion and discoloration of the copper foil surface when the polyamic acid varnish is applied to the copper foil as described in the foregoing, as well as the formation of copper ions which bring about electrical migration in the flexible printed wiring board.
Accordingly, an imidazole-based rust inhibitor (e.g., Adekastub CDA-1; manufactured by Asahi Denka) is generally added to polyamic acid varnishes which are used upon manufacture of flexible printed boards.
However, conventionally used rust inhibitors dissolve poorly in the polyamic acid varnish, and furthermore, excessive blooming occurs on the polyimide surface under the high temperature heating during imidation, resulting in a deterioration in the adhesive strength of the polyimide with respect to the copper foil. Moreover, the rust-inhibitor is scattered during imidation, forming a vapor residue substance that deposits on the imidation process apparatus and on the flexible printed board products and contaminates them.
In an attempt to resolve the aforedescribed problems in the prior art, it is an object of the present invention to provide a polyamic acid varnish composition which is free of problems such as blooming or vapor residue substance formation occurring during the polyamic acid imidation, and electrical migration, and furthermore, from which it is possible to form a polyimide insulating layer which demonstrates satisfactory adhesive strength with respect to copper foil.
The present inventors perfected the present invention based on the discovery that a polyamic acid varnish composition, which is obtained by adding a specific imidazolyl-diaminoazine into a mixture which contains a polyamic acid obtained by the addition polymerization of an aromatic diamine and an aromatic acid dianhydride in a solvent, can achieve the aforesaid objective.
In other words, the present invention provides a polyamic acid varnish composition, comprising a polyamic acid obtained by the addition polymerization of an aromatic diamine and an aromatic acid dianhydride, and a solvent, wherein said polyamic acid varnish composition contains as an additive an imidazolyl-diaminoazine as represented by the Formula (1) 
(In the formula, A is an imidazolyl group as represented by Formulae (1a), (1b) or (1c). 
R1 is an alkylene group, and m is 0 or 1. R2 is an alkyl group and n is 0, 1, or 2. R3 and R4 are alkylene groups, and p and q are respectively 0 or 1. B is an azine, diazine or triazine residue.).
Moreover, the present invention provides a flexible printed board having a polyimide insulating layer furnished on a metal foil, wherein the polyimide insulating layer is formed by forming a film of the aforementioned polyamic acid varnish composition on the metal foil, followed by imidizing.